This invention relates to thin film formation technology suitable for manufacturing color filters or display elements such as LEDs (light emitting diodes) or EL (electroluminescence) devices which use organic semiconductor films.
More particularly, the present invention relates to substrates, a thin film formation method, and thin film elements used in forming patterns on substrates when thin films having different properties are formed on the same substrate. The present invention also relates to a method of forming thin films on which fine patterning is required, wherewith it is both easy to form thin film layers using an ink jet process and possible to form flat thin film layers. The present invention further relates to a surface modification method for performing superfine patterning wherewith a liquid thin film material is deployed in areas enclosed in banks formed on a substrate, using an ink jet method or spin coating, etc., as well as to a method that employs this surface modification method in forming thin films and to both a display element comprising such a thin film and a manufacturing method therefor.
In recent years, technology has been under development for obtaining functional devices by forming prescribed patterns by applying thin films having differing properties on the same substrate. One promising method therefor uses an ink jet process in forming patterns with different thin film patterns on the same substrate. When an ink jet process is used, however, a problem arises at the process surface in that the different thin film materials become mixed on the substrate. In more specific terms, the technology used employs an ink jet process in applying an organic semiconductor material in producing EL devices or other display elements, or a colored resin or other thin film material in producing color filters, but, when the ink jet process is used to deploy a liquid material when forming a thin film pattern, the liquid material that is discharged flows over into adjacent pixels. This presents a problem.
What is commonly done to overcome problems such as this is to provide protruding partitioning members (called “banks” or “risers”) to partition off different thin film areas, and then to fill the areas enclosed by these partitioning members with the liquid materials constituting the different thin films. In the display element example noted above, a method is adopted wherein partitioning members are provided to partition off the various pigment areas, and filling the areas enclosed by the partitioning areas with the materials that configure the pixels.
In recent functional devices, and particularly in display elements, thinness is generally demanded, and, despite the fact that this places limitations on the height of the partitioning members, the volume of liquid material deployed in the areas enclosed by the partitioning members is far greater than the volume remaining after film fabrication.
For this reason, problems arise in terms of poor balance between the size of the liquid droplets discharged into the areas enclosed by the partitioning members and the surface areas both of the partitioning member surfaces and of the areas enclosed therein. This problem will now be described further.
In cases where the partitioning members exhibit liquid affinity or wettability relative to the liquid material that is the thin film material to be deployed, the desired film thickness cannot be obtained in the final thin film even when partitioning members are present since [the liquid material] is pulled by the partitioning members. If the volume of liquid material is made greater, moreover, the liquid material readily flows out into the adjacent areas.
Nevertheless, the surfaces of the areas enclosed by the partitioning members need to exhibit strong liquid affinity and wettability relative to the liquid material so that the liquid material will uniformly wet and spread over those surfaces. If that is not the case, the liquid material will not wet and spread over the areas enclosed by the partitioning members, and, with display elements such as EL devices, in particular, color loss and color irregularity will develop in the pixels.
To deal with problems such as these, technology is proposed in Japanese Patent Laid-Open No. 09-203803 and Japanese Patent Laid-Open No. 09-230129, as published, for example, wherein surface treatments are employed to make the upper portions of the partitioning members liquid-repellant and to make the other portions exhibit liquid affinity.
In all of these examples of the prior art, a layer composed of a liquid-repellant material (a layer made of a fluorine compound) is formed on the upper surface of the partitioning members. In Japanese Patent Laid-Open No. 09-203803, as published, technology is cited wherein a layer exhibiting non-affinity is coated onto the upper portions of the partitioning members, and the surfaces of the areas enclosed by the partitioning members are treated with a hydrophilic base-surface-active agent. In Japanese Patent Laid-Open No. 09-230129, as published, technology is cited for giving the recessed portions enclosed by the partitioning members affinity by additionally exposing them to UV radiation. The theoretical background for this is set forth in “International Display Research Conference 1997”, pp 238-241.
However, even when water repellency in the upper surfaces of the partitioning members and liquid affinity in the areas enclosed by the partitioning members are to some degree realized, in cases where the liquid material is applied using an ink jet process, for example, if the size of the liquid droplets discharged is extremely large or small relative to the surface area of the surfaces of the partitioning members noted above or of the areas enclosed thereby, or if the balance therebetween is otherwise very poor, it is known that the liquid material is not accurately deployed in the areas coated, so that high-precision patterning becomes impossible. When, for example, the size of the liquid droplets noted above is larger than the areas enclosed by the partitioning members to too great an extent, the liquid droplets cross over the partitioning members, and, when the upper surfaces of the partitioning members are narrow, the liquid droplets spill over into areas adjacent to the areas being coated.
In cases such as this, when there is an unsuitable relationship between liquid droplet size and the surface area of the areas enclosed by the partitioning members, what happens is that, due to the problems noted earlier, liquid thin film materials become mixed together in the areas enclosed by the partitioning members, and film thickness variation develops in each thin film that is formed.
Problems also arise which relate to the affinity of the partitioning members toward the liquid thin film material when that thin film material is deployed in the areas demarcated by the partitioning members.
The behavior of the liquid thin film material deployed differs according as to what sort of wettability (affinity) toward the liquid thin film material is exhibited by the partitioning members or the areas enclosed by the partitioning members. As noted earlier, when the surfaces of the partitioning members exhibit affinity (hydrophilic property) toward the liquid thin film material, and the volume of the material deployed exceeds the height of the partitioning members, that liquid thin film material will readily flow over into neighboring areas enclosed by partitioning members even when such partitioning members exist. Conversely, when the surfaces of the partitioning members exhibit a proper degree of non-affinity (water repellency) toward the liquid thin film material, that liquid thin film material will not flow over into the neighboring areas enclosed by partitioning members even when the volume of material deployed exceeds the height of the partitioning members, due to the surface tension of the material.
There are also more specific substrate surface modification methods, such as, for example, those described in the previously cited Japanese Patent Laid-Open No. 09-203803 and Japanese Patent Laid-Open No. 09-230129, as published, and also in Japanese Patent Laid-Open No. 09-230127, as published. That is, specifically, technology involving a method for subjecting bank surfaces to an ink-repellency treatment with a fluorine compound (Japanese Patent Laid-Open No. 09-203803, as published), an etching treatment method (Japanese Patent Laid-Open No. 09-230127, as published), and ink-affinity treatment using energy irradiation (Japanese Patent Laid-Open No. 09-230129, as disclosed).
Nevertheless, when member surfaces are made ink-repellent using a fluorine compound or members are formed using a fluorine compound material, in particular, the bonding strength between the fluorine-based material and the underlying layer or underlying substrate forming the members becomes poor, which presents problems in terms of applications to bank-forming technology. Even if the members, and particularly the banks themselves, are formed with an ink-repellent fluorine compound material or the like, residue develops in the bank areas after patterning by photolithography, whereupon there is a danger of the ink affinity of the bank surfaces being impaired.
In the prior art described in the foregoing, moreover, the application, drying, and removal of materials exhibiting non-affinity are necessary just to impart non-affinity to the upper portions of the partitioning members, whereupon the number of process steps inevitably becomes large. Also, when UV irradiation is performed, there is a tendency for affinity with many materials to develop. There has been a tendency for a slight affinity to develop due to UV irradiation, even when the material is one exhibiting non-affinity, thereby negating the effectiveness of the non-affinity treatment. In Japanese Patent Laid-Open No. 09-230129, as published, in particular, there is a provision to the effect that the degree of affinity be controlled by subjecting both the front and back sides to UV radiation, but, in terms of controlling the affinity between non-affinity and affinity, it is not specified how the various angles of contact relative to the liquid thin film material should be established.
When the liquid repellency of the partitioning members is strong, moreover, the liquid of the thin film material is repelled by the side walls of the partitioning members, wherefore the thickness after film formation becomes thick in the center portions of the areas enclosed by the partitioning members and thin about the peripheries thereof. This results in color irregularities in the pixels in the display elements. In EL devices, in particular, shorts readily develop, leading to reduced reliability.
When the surfaces of the partitioning members are subjected to a liquid-repellency treatment and affinity (liquid-affinity) is imparted to the side surfaces thereof, a thin film material is provided wherewith the thickness after film formation does not become thin about the peripheries of the areas enclosed by the partitioning members. Nevertheless, because most of the liquid of the thin film material is pulled to the side surfaces of the partitioning members, not only does the film thickness become greater in the lower skirt portion of the thin film, that is, in the portions in contact with the substrate, but neither is it difficult to control the film thickness.
There are known methods of modifying the surface energy (wettability) of an organic substance which involve performing a plasma process. One example of such a surface modification method is that described in Japanese Patent Laid-Open No. 63-308920/1988, as published. In the surface modification method set forth in this publication, the organic substance surface is treated with a mixed gas plasma containing a fluorine-based gas and gaseous oxygen, and the surface energy of the organic substance is controlled by varying the mixture ratios between the mixture gasses.
Methods involving UV irradiation or oxygen plasma treatment are also well known as procedures for making the surfaces of organic substances such as glass or indium tin oxide (ITO) hydrophilic.
No technology has been reported, however, for simply and rigorously controlling the wettability of each material in the substrate by plasma treatment or UV irradiation in cases where a pattern of layers constituted by organic or inorganic substances is formed on the same substrate. With methods wherein ink repellency is imparted by mixture-gas plasma treatment to an organic substance surface or the surface of a member formed of an organic substance, problems arise, such as being unable to impart ink repellency efficiently, or that the ink repellency of the surface is transient, or having to use a 10 heat treatment, so that the ink repellency deteriorates with the passage of time.
In cases where an ink-affinity treatment is performed using energy irradiation, there is a danger of impairing the ink repellency of the bank surfaces, and it is very difficult to simultaneously achieve both bank surface ink repellency and bank surface ink affinity.
In methods for forming thin films in prescribed patterns where different thin film materials are provided, and particularly in methods for forming thin films wherein liquid thin film materials are deployed in areas enclosed by partitioning members (banks) formed on a substrate, the proper-control of wettability (ink repellency and ink affinity) in the banks and depressions is critical. If the banks do not exhibit ink repellency, not only will ink residue develop on the banks, but, in cases where different liquid thin film materials are deployed in adjacent depressions divided by a bank, those different liquid thin film materials will overflow the bank and be mixed together. When this happens, it is not possible to form thin films having the desired characteristics.
Examples of the formation of thin films using different liquid thin film materials in adjacent depressions divided by banks include color organic EL devices and color filters used in liquid crystal display elements, etc. When these devices are manufactured, however, the banks must exhibit ink repellency and the areas enclosed by the banks, that is, the surfaces of the ITO or glass substrate, must exhibit ink affinity. If the depressions do not exhibit ink affinity, the wetting and spreading within the pixels will be poor, causing color loss and film thickness irregularity.
With the methods described in the foregoing, moreover, in addition to the ink-repellency treatment, an ink-affinity treatment process is also necessary in the pixel areas, that is, in the depressions. Thus these methods involve difficulties in that controlling the ink supplied is difficult and in that the number of process steps becomes large.